Echo cancelers are commonly used to compensate for both electrical and acoustic echos in communications systems. Typical echo cancelers use well known adaptive filtering algorithms to construct a model of the echo chapel which is excited by the same signal that is delivered to the actual echo channel. In some instances, the echo path can be time varying, for example, when a person or some other object moves about in an acoustic environment where an acoustic echo is being canceled. In such situations a large adaptation step size is often used to improve the convergence speed of the adaptive filtering algorithm used in the echo canceler. Use of a large step size, however, has detrimental effects on the overall performance of the echo canceler and any resulting residual echo, and tends to result in a less stable echo canceler. In order to effectively cancel acoustic echos, it is also desirable to employ an echo canceler having a "long" impulse response in order to model the echo duration found in typical rooms or the like. Use of such "long" echo cancelers having a long impulse response synthesis capability usually means slow convergence times of the adaptive filtering algorithms used in the echo canceler. Thus, when movement occurs in the room, or the impulse response of the echo path changes, the adaptive filtering algorithms used in the echo canceler have difficulty "reconverging" to the new echo path impulse response.
One technique for recovering from echo path changes includes the use of recursive update algorithms, which are known to converge faster than the conventional LMS technique. An arrangement disclosed in an article by V. A. Margo et al. entitled "Multiple Short-Length Adaptive Filters For Time-Varying Echo Cancellation", 1993 IEEE International Conference on Acoustics, Speech, and Signal Processing, Apr. 27-30, 1993, pages I-161-I-164, deals with sparse echo path responses by employing multiple separated echo cancelers that are spaced in time along the echo path. Unfortunately, acoustic echos can not in general be viewed as having a sparse impulse response. It has also been proposed to use a larger step size on the coefficients of the adaptive filtering algorithm being used that are large and a smaller step size on smaller coefficients, along with several other variants.